Fiber lasers and amplifiers with double-cladding ytterbium-doped fibers as a gain medium have the characteristics of high output power, high electro-optic conversion efficiency, high beam quality and the like. However, as the requirement for the laser output power continuously increases, a single fiber laser is difficult to meet the application requirement of several kilowatts to hundreds of kilowatts, and the outputs of a plurality of fiber lasers must be combined into a single laser output by a beam combination method. The outputs of a plurality of fiber lasers manufactured by using large-mode-area fibers with polarization-maintaining characteristics can be coherently combined, so that the laser power is greatly improved.
At present, a polarization-maintaining large-mode-area gain fiber generally adopts a panda-type fiber design. Please refer to FIG. 1, which is a structural schematic diagram of a panda-type polarization-maintaining large-mode-area gain fiber. As shown in FIG. 1, the refractive index of a fiber core 1 is higher than that of a fiber cladding 3, and the refractive index of the fiber cladding 3 is higher than that of stress-applying members 2. However, the panda-type polarization-maintaining large-mode-area gain fiber has the following shortcomings: (1) the diameter of a fiber preform rod should be large enough for hole-drilling and rod insertion to accommodate practical size stress rods, and meanwhile, the machining procedures of drilling, polishing and the like add process complexity and may introduce impurities; (2) the refractive index of the stress-applying members is lower than that of the cladding, which only achieves a polarization-maintaining effect but is usually not part of the waveguide design of the core; (3) since helical rays in a circular cladding cannot be absorbed by the core, low pump absorption is resulted; and (4) for the large mode area fiber designs with small cladding-to-core diameter ratios, the physical space for stress-applying members is limited, so is the ability to achieve high birefringence.
Thus, the panda-type polarization-maintaining large-mode-area gain fiber in the prior art has the technical problems of manufacturing complexity, low birefringence and low pump absorption.